In a vehicle power steering system, a pump provides power steering fluid at an elevated pressure for power assist to the vehicle operator in turning a hand-wheel to steer the vehicle. Most such systems use the vehicle engine itself, through a pulley and belt arrangement, to drive the pump. However, it is also known to use an electric motor to drive the pump independently of the vehicle engine in an electro-hydraulic power steering system; and this allows the pump to be driven at a lower speed for significant energy savings when no power assist is required, such as when the vehicle is proceeding in a straight line or is standing still.
A basic arrangement for an electro-hydraulic power steering system is described in European Patent Application 0053297 A1, published Jun. 9, 1982. The publication describes a pump 4 driven by an electric motor 6, with operating current provided from a battery 11 under control of a power regulator 12. Demand for power assist is indicated by a counter-pressure of the power steering fluid, which increases motor load and thus motor current. A controller 8 varies electric power to motor 6 in response to a motor current sensor 7 as required to meet the demand.
However, the basic premise of this system--that the power steering load is represented by the electric current in the pump driving motor or the hydraulic pressure at the pump outlet--is only approximately true. A portion of the motor's power is used to overcome energy losses in the pump/motor assembly and the fluid conduits of the power steering system. These losses include eddy current and hysteresis losses in the motor, viscous and Coulomb friction in the motor and pump, and viscous losses in the hoses, control valve and gear; and they vary significantly with the temperature of the power steering fluid and the rotational speed of the electric motor, which is preferably submerged with the pump in the power steering reservoir. Over a large range of power steering load, such inaccuracy presents little problem, particularly if the controller provides closed loop control of motor speed. However, in a switching region at the low end of the full range of power steering load, where the motor and pump are switched between a low speed standby mode for energy savings when minimal power assist is required and the significantly higher motor speed required for high power assist, greater accuracy is required.
In the switching region, two requirements for such a power steering system compete with each other. The pump motor is preferably switched from low speed standby to a high speed at a low value of power steering load to avoid "pump catch," in which the pump does not provide sufficient flow soon enough to meet a quickly increasing demand from the power steering system. However, the lower the switch-point, the less time will be spent in standby and the lower will be the energy savings achieved. Thus, the switch point must be carefully chosen to provide the best combination of benefits in view of these two competing considerations.
The use of hysteresis in the switching prevents unnecessary cycling but also aggravates the problem in the selection of switch points, since such hysteresis requires that the upward switch point, which is desirably low, be greater than the downward switch point, which is desirably high. The design of the power steering system is thus a compromise which provides carefully chosen, specific switch points to achieve the best combination of benefits for a particular vehicle. This combination of benefits will not be achieved, however, without significant accuracy in the operation of the system relative to these specific switch points; and this requires accuracy in the switching region in the derivation of power steering fluid pressure from a power steering load signal.